Added working support for private storage (#16903)

* Fixed file corruption bugs in private storage code.

* Restoring fixed test case.

* Implemented test suite for utils/json.py

* Added new unit test cases for util/json.py

* Dixed formatting nags

* Fixed more nags from the Hound

* Added doc strings to some very short functions

* Fixing lint's complains about my choice of parts of speach. Sigh.

* Moved atomic save operations down into util/json.py so that all benefit.

Added extra clean-up code to ensure that temporary files are removed in
case of errors.
Updated emulated_hue unit tests to avoid errors.

* Apparently 'e' is not allows as a variable name for an exception...

tests/util/test_json.py

@@ -0,0 +1,75 @@
+"""Test Home Assistant json utility functions."""
+import os
+import unittest
+import sys
+from tempfile import mkdtemp
+
+from homeassistant.util.json import (SerializationError,
+                                     load_json, save_json)
+from homeassistant.exceptions import HomeAssistantError
+
+# Test data that can be saved as JSON
+TEST_JSON_A = {"a": 1, "B": "two"}
+TEST_JSON_B = {"a": "one", "B": 2}
+# Test data that can not be saved as JSON (keys must be strings)
+TEST_BAD_OBJECT = {("A",): 1}
+# Test data that can not be loaded as JSON
+TEST_BAD_SERIALIED = "THIS IS NOT JSON\n"
+
+
+class TestJSON(unittest.TestCase):
+    """Test util.json save and load."""
+
+    def setUp(self):
+        """Set up for tests."""
+        self.tmp_dir = mkdtemp()
+
+    def tearDown(self):
+        """Clean up after tests."""
+        for fname in os.listdir(self.tmp_dir):
+            os.remove(os.path.join(self.tmp_dir, fname))
+        os.rmdir(self.tmp_dir)
+
+    def _path_for(self, leaf_name):
+        return os.path.join(self.tmp_dir, leaf_name+".json")
+
+    def test_save_and_load(self):
+        """Test saving and loading back."""
+        fname = self._path_for("test1")
+        save_json(fname, TEST_JSON_A)
+        data = load_json(fname)
+        self.assertEqual(data, TEST_JSON_A)
+
+    # Skipped on Windows
+    @unittest.skipIf(sys.platform.startswith('win'),
+                     "private permissions not supported on Windows")
+    def test_save_and_load_private(self):
+        """Test we can load private files and that they are protected."""
+        fname = self._path_for("test2")
+        save_json(fname, TEST_JSON_A, private=True)
+        data = load_json(fname)
+        self.assertEqual(data, TEST_JSON_A)
+        stats = os.stat(fname)
+        self.assertEqual(stats.st_mode & 0o77, 0)
+
+    def test_overwrite_and_reload(self):
+        """Test that we can overwrite an existing file and read back."""
+        fname = self._path_for("test3")
+        save_json(fname, TEST_JSON_A)
+        save_json(fname, TEST_JSON_B)
+        data = load_json(fname)
+        self.assertEqual(data, TEST_JSON_B)
+
+    def test_save_bad_data(self):
+        """Test error from trying to save unserialisable data."""
+        fname = self._path_for("test4")
+        with self.assertRaises(SerializationError):
+            save_json(fname, TEST_BAD_OBJECT)
+
+    def test_load_bad_data(self):
+        """Test error from trying to load unserialisable data."""
+        fname = self._path_for("test5")
+        with open(fname, "w") as fh:
+            fh.write(TEST_BAD_SERIALIED)
+        with self.assertRaises(HomeAssistantError):
+            load_json(fname)